Clean Up and Sync Your Git Fork
If you want to start with your fork from scratch while remaining the current upstream/main as your base use this.
There is a difference between Syncing a fork and Cleaning up a fork.
Clean up your fork from the command line
You can also just delete your fork on the web UI and create a new fork.
Syncing a fork from the command line
GitHub You gives you also the opportunity to sync a fork from the web UI.